On 15 February 1999 an air traffic controller in Scotland noticed something strange on his radar screen. A bright blip on his screen suggested there was a very large object travelling at 3,000mph over the Scottish coastline heading south-west to Belfast.

The size of the blip suggested the object was 10 miles long and two miles wide. Two minutes later the object disappeared from the radar screen.

Three months earlier, MoD documents record that a commercial pilot flying over the Midlands reported an unusual object travelling at 'very high speed' with a very bright strobe light flashing once every 20 seconds.

Although the two incidents were unrelated, both were reported to a little-known department in the Ministry of Defence, known as Secretariat (Air Staff) 2a. This is the secretive section in Whitehall which collates reports of unidentified flying objects that cross British airspace.

Whitehall has traditionally treated reports of UFO sightings as highly classified and only released information to the public after 30 years. But the parliamentary Ombudsman insisted that the MoD hand this information to Colin Ridyard, a research chemist from Wales.

Dr Ridyard had been seeking information relating to UFO sightings by pilots or radar operators between July 1998 and July 1999. Initially the MoD refused on the ground it would be too expensive. But after the intervention of the ombudsman, Michael Buckley, the MoD agreed to release the information as a one-off exercise for £75. The Ministry handed two reports to Ridyard, yet official information from the Civil Aviation Authority suggests there had been additional sightings. During the same period the CAA said it reported two more UFO sightings to the MoD, neither of which the Ministry disclosed.

According to official CAA reports, in the same month that a radar picked up an enormous object flying across Scotland, a pilot flying over the North Sea became startled when his aircraft became illuminated by an 'incandescent' light. Three other aircraft in the area reported seeing a ball of light moving at high speed. Air traffic controllers reported there were no strange aircraft in the area, but five minutes later an operator at a weather station picked up a fast-moving object on his radar.

The other incident which CAA reported to the MoD occurred in June 1999 when the pilot of a B757 flying over the North Sea reported an unidentified military-looking aircraft passing close by in the opposite direction. Nothing was seen on the plane's radar or by air traffic controllers. The MoD told the Authority that there were no military aircraft known to be in that area at the time.

Although an MoD spokeswoman would not discuss individual sightings, she said all these events had perfectly normal explanations. 'Some-times radars have spurious readings caused by military aircraft in the vicinity, and radar-jamming facilities and bright lights on the underside of aircraft can be caused by events on the ground.'

In a letter to one of Ridyard's local MPs, Defence Minister John Spellar said: 'My department has no interest or role with respect to UFO/flying saucer matters or to the question of the existence or otherwise of extraterrestrial life forms - about which we remain open-minded.'

However, declassified gov ernment documents in the Public Records Office from June 1965 reveal that 'it was official MoD policy to play down the subject of unidentified flying objects and to avoid attaching undue public attention or publicity to the subject... as a result we have never had any political pressure to mount a large-scale investigation'. Other documents from that time state: 'The press are never to be given information about unusual radar sightings_ and [unusual visual] sightings are in no circumstances to be disclosed to the press.'

But Ridyard said: 'This is not about little green men, but about freedom of information. It is clear there are many strange incidents that happen in the British skies that are kept secret. There may be issues of aircraft safety or natural phenomena, but by keeping this information secret these incidents cannot be scrutinised by the public or the scientific community.'

One of the most infamous incidents relating to a UFO sighting in Britain only came to light through US freedom of information legislation. This revealed that in December 1980 three security patrolmen investigating a potential air crash near the US Air Force base in Suffolk saw a strange glowing triangular object hovering in the forest near the base which had a 'pulsing red light on top and blue lights underneath'.

An official report by Lieutenant-Colonel Charles Halt , the deputy base commander, included a description of the events and stated that the next day three depressions were found in the forest where the object was discovered which showed radiation readings. Later that night three star-like objects were seen in the sky moving 'rapidly in sharp angular movements'.